# A sample of compound X is found to contain 12.3 g of O, 18.4 g of C, and 25.7 g of N. The Law of Definite Proportion says if it weighs 90.3 g, how many grams of carbon are there?

Oct 9, 2017

There are 29.5 g of carbon.

#### Explanation:

The mass of the first sample is the sum of the masses of its elements.

$\text{Mass of sample" = "(12.3 + 18.4 + 25.7) g" = "56.4 g}$

According to the Law of Definite Proportions, every sample of $\text{X}$ will contain 18.4 g of $\text{C}$ in 56.4 g of $\text{X}$.

Thus, if you have a 90.3 g sample of $\text{X}$,

$\text{Mass of C" = 90.3 color(red)(cancel(color(black)("g X"))) × "18.4 g C"/(56.4 color(red)(cancel(color(black)("g X")))) = "29.5 g C}$